U.S. Secretary of State John Kerry held more than four hours of talks with Russian Foreign Minister Sergei Lavrov, trying to find new agreement on military operations in Syria and movement toward ending the five-year-old conflict.

The July 15 meeting came a day after Kerry held similar talks with President Vladimir Putin, during which Kerry "expressed concern about repeated violations of the cessation of hostilities by the Syrian regime," according to a State Department spokesman.

The Obama administration has repeatedly expressed frustration with Russia’s military intervention in Syria, which has largely been aimed at bolstering the regime of President Bashar al-Assad, a longtime ally of Moscow.

During a break in the talks, Kerry and Lavrov participated in a small joint ceremony at the French Embassy in Moscow, laying flowers outside its entrance to commemorate the victims of the truck attack in Nice, France the night before. An unnamed Russian official told the state news agency TASS that the two diplomats would resume their meeting later July 15.

The Obama administration has repeatedly expressed frustration with Russia’s military intervention in Syria, which has largely been aimed at bolstering the regime of President Bashar al-Assad, a longtime ally of Moscow.

Since launching its air campaign in September, Russian forces have hit Islamic State fighters and groups with ties to terrorist organizations, just as the U.S.-led coalition has.

But Russia has also targeted moderate rebels, some trained by the United States and its allies, and it has done little to rein in Syrian forces from hitting civilian areas with weapons like barrel bombs.

After Kerry’s meeting with Putin, State Department spokesman John Kirby told reporters in Moscow that the U.S. diplomat stressed to Putin that "diplomatic efforts could not continue indefinitely" in the absence of "concrete, near-term steps."

The U.S. administration’s efforts to engage Russia on Syria have been met with deep skepticism in many policy circles in Washington, who are distrustful not only of Moscow’s intentions in Syria, but also its policies in other places like Ukraine and along the borders with NATO member countries.

Ahead of Kerry’s visit, The Washington Post reported that he was bringing to Moscow a major proposal from President Barack Obama on cooperation in Syria.

The proposal, according to the newspaper, would be a new military command-and-control headquarters that would house U.S. and Russian military officers, intelligence officials, and subject-matter experts.

In exchange for U.S. cooperation, the Russians would pressure Syrian President Bashar al-Assad to stop bombing moderate militant groups and civilians, and allow unfettered aid to besieged, rebel-held areas.

Washington also wants Russia's help to start a political transition that would ultimately end the Assad family’s four-decade reign.

According to The Associated Press, opposition to the latest Syria plan is shared by a significant number of officials at the State Department, Defense Department, and in the U.S. intelligence community.

And a so-called “dissent cable” signed by 51 State Department officials last month showed a substantial part of the diplomatic establishment believing a U.S. military response against Assad was necessary.

That wariness has been deepened by Russian air strikes in the past week against U.S.-trained rebel camps.Last month, Russian planes hit a rebel encampment despite advance warnings from U.S. officials, and then returned for a second strike when U.S. jets left the area to refuel.

A U.S. official with access to classified intelligence reports told RFE/RL the incident was “very, very, very serious,” and said the potential for a clash between a U.S. and Russian jets had been high.